1. Field of the Invention
The present invention relates to a semiconductor device and a package structure for a semiconductor device and a power inverter having a semiconductor device.
The present invention relates, in particularly, to a semiconductor device which is able to switching control according to the gate current such as a gate turn-off thyristor and a package structure for a semiconductor device such as a gate turn-off thyristor and a power inverter having a semiconductor device such as a gate turn-off thyristor.
2. Prior Art
A gate turn-off thyristor (hereinafter it is given an abbreviated of "GTO") is a current control type semiconductor device which is able to switching control according to the gate current. In this GTO, a high pressure withstanding performance and a large current performance are attempted to make progress and, in accompany with this, a GTO pellet and a package structure for enclosing the GTO pellet are attempted to make scale-up.
FIG. 16 is a cross-sectional view showing a conventional flat type package structure of the semiconductor device of GTO. A GTO pellet 10 is pressed under pressure in contact with a cathode post 22 of a package structure through a cathode buffer 32 and further is pressed under pressure in contact with an anode post 21 through an anode buffer 31.
The gate current for performing the turn-on and the turn-off of GTO passes a gate lead 23 from a control circuit (not shown in figure) and is supplied to the GTO pellet 10 through a gate pressure contacting portion 33. The gate pressure contacting portion 33 is pressed under pressure by the cathode electrode 22 through a belleville spring member 35 etc. and further is pressed under pressure in contact with the GTO pellet 10.
The gate lead 23 and the gate pressure contacting portion 33 have to electrically insulate from the cathode electrode 22 and so that the gate lead 23 and the gate pressure contacting portion 33 are covered by an insulating material member such as teflon etc..
A gate draw-out region or a contact portion of the gate electrode of the GTO pellet 10 isotopically supplies the gate current to each of plural units GTO and in generally is provided on a central portion of the GTO pellet 10 so as to draw out the gate current.
The potential of the gate is based on the cathode electrode 22 as the standard. As a result, as shown in figure, an auxiliary cathode lead 24 is connected to a cathode flange 42 which is electrically connected to the cathode post 22.
The gate current is given the potential at a portion between a gate G and an auxiliary cathode AK according to a gate power supply (not shown in figure) and the electricity is conducted. With the above stated package structure, the turn-on and the turn-off of GTO is performed.
In the above stated conventional package structure for enclosing GTO, when the diameter of the GTO pellet 10 increases in accompany with the large current performance of GTO, there occurs a problem in which the maximum turn-off current during the turn-off operation is lowered and the unit GTO is easily destroyed. This fact will be explained referring to the figures from FIG. 17 to FIG. 19.
FIG. 17 is an operation explanation view showing the current flow during the "on" period state of the semiconductor device of GTO. So as to simplify, it will pay an attention to merely two units GTO comprised of one unit GTO of GTO1 and another unit GTO of GTO2. For simplification of the explanation, the GTO pellet 10 only comprises those two units GTO composed of the unit GTO1 and the unit GTO2.
In this package structure of the semiconductor device of GTO, the unit GTO1 is provided on a region 11 which is most adjacent portion to the gate pressure contacting portion 33, besides the unit GTO2 is provided on a region 12 which is existed at the most remote portion from the gate pressure contacting portion 33.
In this figure, an anode is indicated by A and a cathode is indicated by K. A gate wiring resistance 13 is exists on a surface of the GTO pellet 10, and each of from reference numerals 61 to 65 indicates an electric resistance of the cathode buffer 32, respectively.
Further, in the cathode electrode 22, an electric resistance 51 exists on a region which contacts to the unit GTO1 through the cathode buffer 32, and an electric resistance 52 exists on a region which contacts to the unit GTO2 through the cathode buffer 32.
In the "on" period state of the semiconductor device, in both units GTO1 and GTO2, the "on" current flows between the anode and the cathode. The "on" current 721 of the unit GTO1 and the "on" current 722 of the unit GTO2 appear.
FIG. 18 is an operation explanation view showing a current flow of a turn-off initial period of the semiconductor device of GTO. This figure indicates a state in which the negative bias is added to the auxiliary cathode lead 24.
At the unit GTO1, the turn-off starts and almost current flown from the anode turns to the gate current 731 and flows out toward an outside of the GTO pellet 10 and a part of the current flows toward the cathode.
The gate current 731 drawn out to the gate passes through a gate power supply (not shown in figure) and further passes through from the auxiliary cathode lead 24 to a cathode flange resistance 81. This gate current 731 turns to the auxiliary cathode current 724 and flows toward the cathode.
Besides, in the unit GTO2, because of the potential grade according to the wiring resistance 13 of the gate, the draw-out gate current 732 is little. Almost current turns to the current of the anode A--the cathode K interval current 722 and only flows toward the cathode.
FIG. 19 is an operation explanation view showing the current flow at a state of the semiconductor device of GTO in which the time goes forward. From the unit GTO2 the draw-out of the gate current 732 becomes large. However, the anode A--the cathode K interval of GTO presents the "on" state and the gate current 732 flows toward the cathode of the unit GTO2.
A part of the gate current 732 passes through the cathode buffer 32 and flows into the unit GTO1 from the cathode and this gate current 732 turns to the gate current 733 of the unit GTO1 and is drawn out to the gate. Accordingly, the cathode current of the unit GTO1 becomes negative at this state.
The gate currents 732 and 733 turn to the auxiliary cathode current 724 through a gate power supply (not shown in figure) and flow into toward the cathode. After that, when the current of the anode A--the cathode K interval current 722 of the unit GTO2 turns off, then the unit GTO as a whole works to turn off.
However, when a diameter of the GTO pellet 10 becomes large, since the gate wiring resistance 13 becomes high, the draw-out of the gate current becomes insufficient. Before the unit GTO2 performs to turn off, the anode current is concentrated in the unit GTO2 and it reaches to destroy of the unit GTO2.
FIG. 20 is a wave-form view of the semiconductor device of GTO in which the structures shown from FIG. 17 to FIG. 19 are readjusted by arranging with the wave forms. Each of the time change of the anode current and the time change of the anode voltage is schematically shown.
A time A in this figure corresponds to the structure shown in FIG. 17 and shows the "on" state. Further, a time B corresponds to the structure shown in FIG. 18 and shows a vicinity of the last phase end of the storage.
Since the unit GTO1 is on the way of the turn-off, however the unit GTO2 is the "on" state, the anode current is concentrated to the unit GTO2 and in the unit GTO as a whole the current continues to flow.
A time A corresponds to the structure shown in FIG. 19 and shows a vicinity of the finish phase of the fall. The unit GTO1 presents the "off" state and the unit GTO2 is on the way of the "off" state. As a result, in the unit GTO as a whole the anode current abruptly decreases.
However, since the unit GTO2 is not completely turn off and the fall is not completely performed, some what fall-in of the current (the small value of the anode current during the fall finish) may be seen but the fall is not completely performed. After that, the anode current raises at a predetermined voltage raise rate.
When the unit GTO2 presents the "off" state, after a lapse of a tail period, by having a wave-form shown in a dot line in this figure it reaches to the "off" state.
However, when the current fall-in at the fall finish time is insufficient, since the "off" state of the unit GTO2 is incomplete, it exceeds the maximum turn-off current of the unit GTO and reaches to destroy of the unit GTO. Then the anode voltage abruptly decreases and further the anode current raises.
As stated in the above, the destroy of the unit GTO causes by a fact in which the unit GTO region remote from the gate pressure contacting portion 33 hardly to present the "off" state by the existence of the gate wiring resistance 13 of the gate.
As disclosed in Japanese patent laid-open publication No. Hei 2-137,371, it has proposed that in response to the distance from a gate pressure contacting portion a life time of each of the unit GTO regions is shorten and easily perform to turn off.
However, in the above stated case, it has a problem that an affect of the time life adjustment appears at the "on" state and a current balance between each of the unit GTO regions at the "on" state becomes wrong. To put it concretely, it is considered that the "on" voltage of a whole GTO pellet increases in comparison with no life time adjustment, for example.
Besides, in Japanese patent publication No. Sho 63-58,376, a gate pressure contacting portion is not arranged at a central portion of a GTO pellet but it is arranged at an intermediate portion between the central portion of the GTO pellet and an outer periphery portion of the GTO pellet, namely an intermediate ring gate system package structure is proposed.
However, in the above case, the package structure for enclosing GTO becomes complicated one, there is an anxious that it is difficult to uniformly press under pressure the unit GTO as a whole. Further, since it exists inevitably the unit GTO region which is most remote portion from the gate pressure contacting portion, thereby a problem is not still solved, such a problem is that the turn-off of GTO hardly is performed at this unit GTO region.